Feasible Coefficient Region Analysis and Dual-Loop Adaptive Feedback Control for Transient Stability of VSG Under Severe Grid Voltage Sag

Aiming at the transient instability and overcurrent issues of the virtual synchronous generator (VSG) under severe grid voltage sag, the accurate and simultaneous control for the phase angle and current of VSG is hard to be achieved without using the fault information. And the requirement of the grid code for the reactive current should be also considered. To address the issues, this paper proposes a non-fault information based dual-loop adaptive feedback control to take transient angle stability, current limitation and the demand of the reactive current of VSG into account. First the large-signal model of VSG with a dual-loop control is built. To design the feedback coefficients, the feasible coefficient region under different fault degrees and cases is analyzed subsequently. It provides reference for the curve fitting, which is further applied in the self-adaptive regulation of the feedback coefficients. Thereby, a dual-loop adaptive feedback control is realized based on an additional reactive power feedback loop. With the proposed control scheme, all of the three control objectives can be achieved without the fault information, since the feedback coefficients are within the feasible coefficient region by the self-adaptive regulation. Finally, the effectiveness and robustness of the proposed control scheme for both VSG and a paralleled system of VSG and grid-following (GFL) converter are validated by the simulation results and the experimental results.